The present invention relates to heavy duty industrial gas turbines and, in particular, to a burner for an industrial gas turbine including a fuel/air premixer enabling high-efficiency operation without producing undesirable air polluting emissions.
Gas turbine manufacturers are currently involved in research and engineering programs to produce new gas turbines that will operate at high efficiency without producing undesirable air polluting emissions. The primary air polluting emissions usually produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen, carbon monoxide, and unburned hydrocarbons. It is well known in the art that oxidation of molecular nitrogen in air breathing engines is highly dependent upon the maximum hot gas temperature in the combustion system reaction zone. The rate of chemical reactions forming oxides of nitrogen (NOx) is an exponential function of temperature. If the temperature of the combustion chamber hot gas is controlled to a sufficiently low level, thermal NOx will not be produced.
One preferred method of controlling the temperature of the reaction zone of a heat engine combustor below the level at which thermal NOx is formed is to premix fuel and air to a lean mixture prior to combustion. The thermal mass of the excess air present in the reaction zone of a lean premixed combustor absorbs heat and reduces the temperature rise of the products of combustion to a level where thermal NOx is not formed.
There are several problems associated with dry low emissions combustors operating with lean premixing of fuel and air. That is, flammable mixtures of fuel and air exist within the premixing section of the combustor, which is external to the reaction zone of the combustor. There is a tendency for combustion to occur within the premixing section due to flashback, which occurs when flame propagates from the combustor reaction zone into the premixing section, or autoignition, which occurs when the dwell time and temperature for the fuel/air mixture in the premixing section are sufficient for combustion to be initiated without an igniter. The consequences of combustion in the premixing section are degradation of emissions performance and/or overheating and damage to the premixing section, which is typically not designed to withstand the heat of combustion. Therefore, a problem to be solved is to prevent flashback or autoignition resulting in combustion within the premixer.
In addition, the mixture of fuel and air exiting the premixer and entering the reaction zone of the combustor must be very uniform to achieve the desired emissions performance. If regions in the flow field exist where fuel/air mixture strength is significantly richer than average, the products of combustion in these regions will reach a higher temperature than average, and thermal NOx will be formed. This can result in failure to meet NOx emissions objectives depending upon the combination of temperature and residence time. If regions in the flow field exist where the fuel/air mixture strength is significantly leaner than average, then quenching may occur with failure to oxidize hydrocarbons and/or carbon monoxide to equilibrium levels. This can result in failure to meet carbon monoxide (CO) and/or unburned hydrocarbon (UHC) emissions objectives. Thus, another problem to be solved is to produce a fuel/air mixture strength distribution, exiting the premixer, which is sufficiently uniform to meet emissions performance objectives.
Still further, in order to meet the emissions performance objectives imposed upon the gas turbine in many applications, it is necessary to reduce the fuel/air mixture strength to a level that is close to the lean flammability limit for most hydrocarbon fuels. This results in a reduction in flame propagation speed as well as emissions. As a consequence, lean premixing combustors tend to be less stable than more conventional diffusion flame combustors, and high level combustion driven dynamic pressure activity often results. This high level dynamic pressure activity can have adverse consequences such as combustor and turbine hardware damage due to wear or fatigue, flashback or blow out. Thus, yet another problem to be solved is to control the combustion driven dynamic pressure activity to an acceptably low level.
Lean, premixing fuel injectors for emissions abatement are in common use throughout the industry, having been reduced to practice in heavy duty industrial gas turbines for more than two decades. A representative example of such a device is described in U.S. Pat. No. 5,259,184, dated Nov. 9, 1993, invented by Richard Borkowicz, David Foss, Daniel Popa, Warren Mick and Jeffery Lovett; and assigned to the General Electric Company. Such devices have achieved great progress in the area of gas turbine exhaust emissions abatement. Reduction of oxides of nitrogen, NOx, emissions by an order of magnitude or more relative to the diffusion flame burners of prior art have been achieved without the use of diluent injection such as steam or water.
These gains in emissions performance, however, have been made at the expense of incurring several problems. In particular, flashback and flame holding within the premixing section of the device result in degradation of emissions performance and/or hardware damage due to overheating. In addition, increased levels of combustion driven dynamic pressure activity results in a reduction in the useful life of combustion system parts and/or other parts of the gas turbine due to wear or high cycle fatigue failures. Still further, gas turbine operational complexity is increased and/or operating restrictions on the gas turbine are necessary in order to avoid conditions leading to high-level dynamic pressure activity, flashback, or blow out.
In addition to these problems, conventional lean premixed combustors have not achieved maximum emission reductions possible with perfectly uniform premixing of fuel and air.
An example of a method for reducing the amplitude of combustion driven dynamic pressure activity in lean premixed dry low emissions combustors can be found in U.S. Pat. No. 5,211,004 dated May 18, 1997, invented by Steven H. Black, and assigned to General Electric Company. The current invention builds upon the principles disclosed in this prior patent by controlling both fuel/air radial profile and fuel injection pressure drop to minimize or eliminate the amplification resulting from the weak limit oscillation cycle.
The current invention is an improvement relative to the prior art in that the unique features of the premixer cause it to achieve performance improvements relative to the prior art in all of the problem areas noted above.
It is an object of the invention to achieve gas turbine exhaust emissions performance that is superior to current technology lean premixed dry low emissions combustor performance at elevated firing temperatures of the most advanced heavy-duty industrial gas turbines. In particular, the emissions of oxides of nitrogen (NOx) are to be minimized without compromising carbon monoxide (CO) or unburned hydrocarbon (UHC) emissions performance. It is another object of the invention to improve upon the resistance to flashback and flame holding within the premixer relative to current technology lean premixed dry low emissions combustors for heavy-duty industrial gas turbine application. It is yet another object of the invention to reduce the level of combustion driven dynamic pressure activity and increase the margin to lean blow out over the entire operating range of the gas turbine relative to current technology lean premixed dry low emissions combustors for heavy duty industrial gas turbines.
These and other objects of the invention are realized through the use of an inlet flow conditioner (IFC) located upstream of the premixer inlet. The IFC improves the air flow velocity distribution through the premixer, which improves the uniformity of the fuel/air mixture exiting the premixer. The premixer is made less sensitive to air flow maldistribution in the flow field approaching the premixer, and the distribution of air flow among burners of a multi-nozzle combustor is made more even through the use of the inlet flow conditioner.
In addition, fuel is injected through the surfaces of air foil shaped turning vanes in the premixer swirler in lieu of the conventional fuel injection tubes, spokes or spray bars of prior art. Fuel injection through the surfaces of the turning vanes minimizes the disturbance of the flow field and does not generate regions where the flow of fuel/air mixture stagnates or recirculates within the premixer. These regions of flow stagnation and/or recirculation, which are characteristic of the more intrusive, less aerodynamic features of prior art fuel injectors, form locations where flame can anchor in the premixer. Elimination of these regions makes it more difficult for flame to propagate into the premixer and for combustion to be sustained within the premixer.
Moreover, radial fuel/air mixture strength distribution control is obtained with two or more independently controllable fuel supplies injected at different locations on the aerodynamic turning vane surfaces. By controlling the relative richness of the mixture from hub to tip shroud on the swirler, dynamic pressure activity level and lean blow out margin can be controlled as the overall combustor stoichiometry is varied to match turbine load.
The invention combines three aerodynamic design innovations to produce a fuel/air premixer for use in the combustion system of a heavy-duty industrial gas turbine, burning natural gas fuel, which provides exceptional performance in the areas of fuel/air mixture uniformity, flashback resistance, and control of combustion driven dynamic pressure activity. The three aerodynamic design innovations are: (1) Inlet air flow conditioning; (2) Fuel injection through the vanes of an air swirler (xe2x80x9cswozzlexe2x80x9d assembly); and (3) Radial fuel/air concentration distribution profile control.
An inlet flow conditioner (IFC) includes a perforated annular shell at the inlet to the fuel/air premixer swirler through which air flowing to the premixer must pass. The pattern of perforations in this shell is designed such that a uniform air flow distribution is produced at the swirler inlet annulus in both the radial and circumferential directions. The pressure drop of the inlet flow condition allows it to produce the desired swirler inlet air flow uniformity even when a non-uniform flow field exists in the plenum surrounding the burner inlet.
The swozzle assembly includes a series of preferably air foil shaped turning vanes that impart swirl to the air flow entering via the IFC. Each air foil contains internal fuel flow passages that introduce natural gas fuel into the air stream via fuel metering holes, which pass through the walls of the air foil shaped turning vane. By injecting fuel in this manner, an aerodynamically clean flow field is maintained throughout the premixer. The flow stagnation and/or separation and recirculation associated with more intrusive fuel injection methods, such as the conventional fuel tubes or spray bars of prior art, are avoided, and this improves the resistance of the premixer to flashback and flame holding.
The purpose of injecting fuel via two separate passages and two sets of injection holes is to provide control over the fuel/air mixture strength distribution in the radial direction. By varying fuel flow split between the passages, optimum radial concentration profiles can be obtained for control of emissions, lean blow out, and combustion driven dynamic pressure activity as machine and combustor load are varied.
Downstream of the swozzle is an annular mixing passage formed between the hub and the shroud. Fuel/air mixing is completed in this passage, and a very uniform mixture is injected into the combustor reaction zone where burning takes place. Emissions generation is minimized because the uniformly lean mixture does not yield local hot zones where NOx is produced. In the center of the premixer is a conventional diffusion flame fuel nozzle, which is used at low turbine load when the mixture from the premixer becomes too lean to burn.